1. Field of the Invention
The present invention relates to a radiation image pickup apparatus including an X-ray image pickup apparatus, and a method of driving the apparatus, and more particularly to a radiation image pickup apparatus for forming an image representing the intensity distribution of radiation such as X-ray, and a method of the apparatus.
2. Related Background Art
In case of forming an image representing the distribution of a radiation such as X-ray transmitted by an inspected object for example in a non-destructive inspection, there may only be obtained a blurred image about the interior of the object, since such image not only represents the linearly transmitted component but also the scattered components, generated in the inspected object transmitting the X-ray. In the medical diagnostic field, there has long been adopted a method of providing a so-called grid, consisting of a plurality of lead plates arranged in a mutually spaced and parallel manner, thereby guiding the straight proceeding component only to a fluorescent plate or the image pickup apparatus for converting the X-ray distribution into the image.
Such grid is generally formed by arranging lead plates into a one-dimensional grating, so that the obtained image bears a striped pattern corresponding to such grating.
Such grating pattern tends to be very conspicuous in the image in case the image is recorded for example on a film. Also such grid, effecting spatial multiplication, shows an effect of modulating the image itself with the frequency of the grid, so that components finer than the frequency of the grid tend to be lost if the grid is present. Also there is recently developed an apparatus capable of directly acquiring the distribution of X-ray with an image pickup apparatus and converting such distribution into a digital image by sampling, and, in such apparatus, the grid image is modulated by the sampling carrier whereby stripes of a frequency different from the grid frequency becomes conspicuous. (This phenomenon may be understood as an aliasing of the spatial spectrum of the grip by the sampling.)
For preventing such phenomena, there have been proposed various methods of moving the grid in a direction perpendicular to the stripes thereof during the X-ray irradiation, thereby reducing the contrast of the grid and extinguishing the stripes.
In the following there will be considered the mode of contrast reduction of the grid stripes by such movement. It is assumed that the grid is limited to a one-dimensional structure and the spectrum is considered in a direction perpendicular to the grid stripes. If the spectrum of the grid is represented by a function G(f) (wherein f is spatial frequency) and the OTF (optical transfer function) of the film, the fluorescent plate which converts the intensity of X-ray into the intensity of fluorescent light or the image pickup apparatus is represented by a function H(f), the spectrum L(f) of the grid which is finally obtained through the fluorescent plate, etc. is represented by the following equation (1): EQU L(f)=G(f).times.H(f) (1)
As the grid can be represented by a periodic function, if the grid pitch is T.sub.g, G(f) can be represented, utilizing Fourrier series development by a group of linear spectra. Since H(f) is a linear filtering mechanism, H(f) can also be represented by a group of linear spectra, and is represented by the following equation (2): ##EQU1## wherein .delta.(f) is the Dillac's delta function, a.sub.n =a.sub.-n and b.sub.n =-b.sub.-n (n being an integer).
The contrast when the grid is stopped can be obtained by determining the spatial contrast through inverse Fourrier conversion of the above equation (2).
If a grid with a spatial pitch Tg of the stripes moves at a constant speed in a direction perpendicular to the stripes, the spatial shape s(x) under the X-ray irradiation of a predetermined amount for a period of movement of m stripes over a point is represented by the following equation (3): ##EQU2## wherein the inverse Fourrier conversion of L(f), namely a shape in a real space is regarded as l(x).
As the frequency characteristics S(f) of s(x) is the product of L(f) and the frequency characteristics of a rectangle of distance mT.sub.g, namely sinc function, it can be represented by the following the equation (4): EQU .vertline.S(f).vertline..varies..vertline.L(f).vertline..times.sin (.pi.mfT.sub.g)/(.pi.mfT.sub.g) (4)
wherein the frequency characteristics represent only the amplitude since the phase is disregarded.
From the equations (2) and (4), it will be understood that, when m is a non-zero integer, the line spectrum component of L(f) overlaps with the zero point of sinc function, whereby .vertline.S(f).vertline. becomes the DC component alone and the stripes of the grid are completely extinguished.
The sinc function (sin.pi.mfT.sub.g /.pi.mfT.sub.g) in the equation (4) always becomes zero (zero point) at f=k/(mT.sub.g) wherein k is a non-zero integer. The equation (2) has only a value at f=n/T.sub.g wherein n is an integer, so that, in the product of the both, when m is a non-zero integer in .vertline.S(f).vertline., the zero point of sinc function coincides with a non-zero f value of the equation (2) to cancel components with f being other than 0. Consequently there is only left the DC component.
FIG. 1 is a graph showing the spatial contrast of the grid image after passing the fluorescent plate in the ordinate, as a function of the number of moving stripes of the grid during the irradiation time in the equation (4) in the abscissa, calculated by the OTF of the fluorescent plate. It is represented in decibels, taking the contrast of the grid itself as reference. As shown in this graph, the entire contrast becomes lower with an increase in the moving distance, and, in the illustrated example, the contrast becomes -40 dB (1/100) or lower with the passing of 11 or more stripes and is therefore in the practically acceptable level. However, since the movement is conducted mechanically, it is very difficult to always move 10 or more stripes in any X-ray irradiation time, and a powerful driving system has to be provided for this purpose. For this reason, it is desired to reduce the contrast of the grid even with the movement of a short distance. In FIG. 1, for example a moving distance in a range A shows a contrast of -40 dB or less even with a moving distance of about 5 stripes. Such range always exists in the vicinity of any non-zero integral value of m. Consequently, the grid contrast can be significantly reduced even with a short distance, by moving the grid by an integral number of stripes corresponding to the X-ray irradiation time.
In order to obtain an X-ray image of high quality in the field of medical diagnosis or non-destructive inspection, since the required amount of X-ray varies depending on the fluctuation of an inspected object such as the human body (for example, body size or inspected region), the optimum X-ray dose (irradiation time) is determined utilizing a device called a phototimer, which measures the amount of X-ray transmitted by the inspected object such as the human body. In such case there is generally employed a method of monitoring the accumulated amount of the X-ray transmitted by the inspected object such as the human body and stopping the X-ray irradiation when a predetermined dose is reached.
Consequently the X-ray irradiation time varies depending on the inspected person, the inspected region or the kind of the inspected object. Therefore, even if the aforementioned grid movement in the range A in FIG. 1 is carried out, such movement cannot be controlled since the irradiation time cannot be known in advance.
Also the X-ray irradiation may not be constant in time, for example due to the influence of fluctuation of the power supply. In such case, the graph shown in FIG. 1 cannot be applied, and it becomes difficult to control and completely extinguish the grid image.
For forming an image representing the distribution of X-ray radiation, the distribution of radiation is converted with a fluorescent plate into an optical intensity distribution, which is recorded as a latent image on a silver salt film and developed, but in recent years there is also proposed a method of converting such optical intensity distribution with an image pickup device into electrical signals, which is then converted into digital data and formed into a digital image. In this method there is also known a system of directly converting the distribution of X-ray radiation directly into electrical signals without employing the fluorescent plate. The aforementioned difficulty with the stripe pattern also occurs in these cases.
In such case, in order to convert the continuous distribution of X-ray radiation, transmitted by the inspected object, into a discrete distribution, there is required spatial sampling in a matrix pattern with a predetermined pitch.
Since such spatial sampling naturally acquires the above-mentioned grid image at the same time, there is in this case generated a drawback of pseudo resolution of the grid image.
More specifically, based on the basic sampling principle, in case the grid having the spectral characteristics L(f) represented by the equation (2) is sampled with a sampling pitch Ts (multiplication of a train of Dillac's delta functions with a pitch T.sub.s), the sampled spectrum L'(f) can be obtained by the convolution calculation with the spectral characteristics of the sampling function, i.e., by the following equation (5): ##EQU3##
Stated differently, even in case of considering only the basic pitch T.sub.g of the grid, the grid spectral frequency by aliasing appears at positions .vertline.1/T.sub.s .+-.1/T.sub.g .vertline.. As an example, if 1/T.sub.g &gt;1/(2T.sub.s), the grid pattern appears at the Nyquist frequency of sampling or lower to generate a low-frequency image, which is mixed with the original image spectrum and cannot be separated therefrom, resulting in a seriously defective image.
Even if the grid is moved to reduce the contrast as mentioned above, the influence of the grid cannot be eliminated completely since the spectral position remains unchanged.
Also in case the sampling frequency is so selected as to prevent the aliasing of the basic pitch, such as 1/T.sub.g &lt;1/(2T.sub.s), the influence of the high frequency components is also strong and the grid stripes cannot be sampled in a stable manner, so that the low-frequency pseudo resolution tends to appear with a high probability.